1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) capable of improving display quality and a method of driving the same.
2. Discussion of the Related Art
A liquid crystal display (LCD) controls the transmittance of a liquid crystal layer through an electric field applied to the liquid crystal layer in response to a video signal to display an image. The LCD is a flat panel display (FPD) that is small and thin and that consumes a small amount of power and is used as a portable computer such as a notebook PC, an office automation apparatus, and audio/video apparatuses. In particular, since an active matrix type LCD in which switching devices are formed in liquid cells, respectively, can actively control the switching devices, it is advantageous to realizing a moving picture.
A thin film transistor (hereinafter, referred to as TFT) illustrated in FIG. 1 is mainly used as the switching device used for the active matrix type LCD.
Referring to FIG. 1, the active matrix type LCD converts digital video data into an analog data voltage based on a gamma reference voltage to supply the analog data voltage to a data line DL and supplies a scan pulse to a gate line GL to charge the data voltage in a liquid crystal cell Clc. Therefore, the gate electrode of the TFT is connected to the gate line GL, the source electrode of the TFT is connected to the data line DL, and the drain electrode of the TFT is connected to the pixel electrode of the liquid crystal cell Clc and one side electrode of a storage capacitor Cst1. A common voltage Vcom is supplied to the common electrode of the liquid crystal cell Clc. The storage capacitor Cst1 charges the data voltage applied from the data line DL when the TFT is turned on to maintain the voltage of the liquid crystal cell Clc to be uniform. When the scan pulse is applied to the gate line GL, the TFT is turned on to form a channel between the source electrode and the drain electrode and to supply the voltage on the data line DL to the pixel electrode of the liquid crystal cell Clc. At this time, the arrangement of the liquid crystal molecules of the liquid crystal cell Clc is changed by an electric field between the pixel electrode and the common electrode to modulate incident light.
When a direct current (DC) voltage is applied to the liquid crystal layer of the LCD for a long time, ions having negative charges move in the one motion vector direction and ions having positive charges move in the other motion vector direction to be polarized in accordance with the polarity of the electric field applied to liquid crystal layer and the accumulation amount of the ions having the negative charges and the accumulation amount of the ions having the positive charges increase with the lapse of time. As the accumulation amount of the ions increases, an alignment layer deteriorates. As a result, the alignment characteristic of the liquid crystal deteriorates. Therefore, when the DC voltage is applied to the LCD for a long time, spots appear in a displayed image and the spots increase with the lapse of time. In order to reduce the spots, a method of developing a liquid crystal material having a low permittivity or of improving an alignment material or an alignment method was attempted. However, such a method requires a large amount of time and cost for developing a material. When the permittivity of the liquid crystal is reduced, another problem of deteriorating the driving characteristic of the liquid crystal occurs. As noted by experiments, the point of time at which the spots appear due to the polarization and accumulation of the ions gets faster as impurities ionized in the liquid crystal layer increase and accelerating factors are large. The accelerating factor are temperature, time, the DC driving of the liquid crystal. Therefore, the spots appear faster and get severe as the temperature is higher or the time for which the DC voltage of the same polarity is applied to the liquid crystal layer is longer. Furthermore, since the shapes and degrees of the spots are different in the panels of the same model that are manufactured through the same manufacturing line, it is not possible to remove the spots by developing a new material or by improving processes.